Fuel performance modeling of high burnup mixed oxide fuel for hard spectrum LWRs

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 408-431).According to the future of the nuclear fuel cycle study at MIT, a reactor with a conversion ratio around one can achieve desired objectives in the long-term sustainability of uranium and reduction of transuranic wastes. This finding relaxes the need for sodium fast reactors (SFR) in a closed-loop nuclear fuel cycle and enables high-conversion light water reactors (HC-LWR) to be used as an alternative. HC-LWRs have two major advantages over SFRs. First, apart from the reactor core, the remaining reactor system can be based on existing LWR technology. Second, extensive operating experience and a proven record of high reliability of LWRs would ease licensing and commercialization processes. Therefore, operating HC-LWRs instead of SFRs may be more economically and technically viable with lower capital and development cost for the near term. This type of reactor is being developed by Hitachi Ltd. under the name of resource-renewable boiling water reactor (RBWR). This study focuses on RBWR-TB2, transuranic burning version of RBWR. To demonstrate that the RBWR-TB2 can operate safely within design constraints and regulatory limits, the thermomechanical behavior of this reactor has been analyzed through fuel performance modeling. Due to its unique design characteristics, several physical phenomena at high temperature and high burnup typically ignored in most LWR fuel performance codes can potentially become active under RBWR's operating conditions. These phenomena involve migration of fuel constituents and fission products, the evolution of O/M ratio with burnup, high burnup structure (HBS) formation, accelerated corrosion, hot pressing, gaseous fuel swelling, hydride precipitation and hydrogen migration in the cladding. Semi-empirical models describing porosity and cesium migration behaviors have been replaced with mechanistic models. All of these phenomena have been successfully implemented in a modified version of FRAPCON-3.5 known as FRAPCON-3.5 EP where EP stands for enhanced performance. The fuel performance comparison between RBWR-TB2 and ABWR fuel rods suggest that because of high axial peaking factors and relatively flat power history, fuel temperature is significantly higher in fissile zones of the RBWR-TB2 leading to various undesirable effects such as excessive fission gas release and cladding deformation. Local fuel burnup in fissile zones of RBWR-TB2 is multiple times higher than that of ABWR leading to excessive fuel swelling, accelerated cladding oxidation, and PCMI at fissile-blanket interfaces. Even if the RBWR-TB2 has to operate under such demanding conditions with a small margin to fuel melting, a steady-state fuel performance analysis still shows that this reactor can operate safely with an acceptable thermo-mechanical performance. In the future optimization of RBWR-TB2 performance, several fuel design strategies are recommended based on a series of sensitivity studies. The sensitivity study on key design parameters indicates that using annular fuel geometry and more hypostoichiometric fuel (lower O/M ratio) could reduce fuel temperature at high burnup. For better resistance to cladding corrosion and PCMI, it is recommended to increase cladding thickness and decrease fuel density.by Yanin Sukjai.Ph. D